Levels have been used for many years to determine the angular orientation in a vertical plane of a surface with the horizontal. Typical levels have an air bubble in a glass tube which lines up with specific marks on the tube when the level is horizontal or in the desired angular orientation. Herman, in U.S. Pat. No. 4,592,147, discloses an electronic level which uses a pendulum containing a thin wire with a weight on its distal end to determine the angular orientation of an object.
The '147 patent describes an electronic level which provides the user with a visual and audible means of indicating when a desired angle, such as horizontal, is reached. The angle detection means consists of a metal sphere suspended by a fine thread (called a "plumb line" by Herman). The thread interrupts a light beam when the desired angle is attained. Light which is provided from a source, such as an infrared LED, passes through a restricting aperture, enters the string pendulum area, then passes through another aperture, and finally is received by a phototransistor. Although this is a functional system, it does not permit tilting the level in a forward or backward direction by an acceptable amount without possibly disabling the pendulum action.
As used herein to describe the angular movement of the level, the term "rotation" pertains to the normal angular displacement of the level when determining a surface angle. The term "tilt" describes the rotation of the level from an end-on view. Furthermore, as used herein, the phrase "angular orientation" means angular orientation relative to horizontal of a line in a predetermined vertical plane. In the Herman patent, this line is defined by the weighted plumb line that hangs freely under gravity. The phrase "desired angular orientation", as used herein, means the direction, relative to horizontal in the predetermined vertical plane, that is desired for the level. The phrase "error angle" (.theta.), as used herein, means the difference, expressed in degrees, between the actual angular orientation of a perpendicular to the surface being leveled, and the desired angular orientation of a vertical or plumb line in the predetermined vertical plane.
When using the Herman level, the structure to be oriented is positioned so that, when the predetermined desired angular orientation of the structure is attained, the plumb line either completely or maximally blocks the light path of a light beam that is directed from one side of the plumb line to the other side. When the plumb line does not completely block the light path, the unblocked light is received by a light receptor that is spaced apart from the source of the light beam. When the plumb line partially or completely blocks the light beam path, correspondingly less light is received by the light receptor. The weight (called a plumb bob by Herman) in the Herman device is constrained to move in a rather narrow channel. If the Herman device is tilted by too large an angle, of the order of five degrees, the plumb bob and the plumb line will bind against a wall of the device and not move freely. The Herman device provides visual and/or aural indication that the desired angular orientation has been achieved and that the actual angular orientation is within a predetermined error angle of the desired orientation. However, the Herman device provides no indication that the desired angular orientation is being approached, except for indicating that the plumb line orientation is within a predetermined error angle, typically 0.2.degree., of the desired angular orientation.
Other leveling devices that use a pendulum or similar approach include U.S. Pat. No. 3,233,235, issued to Wright, which uses a knife edge magnet to close an electric circuit when the desired angular orientation is achieved; and U.S. Pat. No. 4,094,073, issued to Parra, which provides for a weighted pendulum to achieve contact in an electric circuit when the desired angular orientation has been achieved. Other electronic leveling device disclosures include U.S. Pat. Nos. 3,371,424, 3,861,052, 4,027,399, 4,077,132, 4,079,521, 4,085,375, 4,110,609, 4,154,000 and 4,182,046 and Australian Pat. No. 17828/82 (lodged Aug. 19, 1983).